Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and, in particular, to an apparatus configured to bind sheets together without the use of a staple or other external device.
Description of the Related Art
Conventionally, some image forming apparatuses, such as copying machines, laser beam printers, facsimile apparatuses, and multifunction peripherals as combinations thereof, have been provided with a sheet processing apparatus configured to perform binding processing on sheets. Generally, such image forming apparatuses bind a sheet bundle with the use of a metallic staple. Such stapling processing allows a plurality of output sheets to be securely bound at a position specified by a user, and therefore is employed in a large number of sheet processing apparatuses.
However, although the stapling processing using a metallic staple allows the sheet bundle to be bound securely, a special tool should be used to release the sheet bundle once it is bound by this processing. Further, this processing requires work to remove the staple before the stapled sheets are put through a shredder. Similarly, when the stapled sheet bundle is recycled, the staple should also be removed, and the sheets and the staple should be separately collected.
Therefore, apparatuses configured to bind sheets without the use of a staple, especially in consideration of recyclability, are proposed among conventional sheet processing apparatuses. These sheet processing apparatuses, for example, include apparatuses configured to perform binding processing on a sheet bundle by a binding unit including V-shaped upper teeth and inverted V-shaped lower teeth (see Japanese Patent Application Laid-Open Nos. 2010-189101 and 2011-201653).
According to these sheet processing apparatuses, after sheets are bundled together and aligned to one another, the lower teeth and the upper teeth of the binding unit are engaged with each other to form an uneven surface on a part of the sheet bundle in a thickness direction to cause respective fibers of the stacked sheets in the sheet bundle to be entangled with one another, thereby binding the sheet bundle. In other words, these sheet processing apparatuses are configured to bind fibrous sheets without the use of a staple. Hereinafter, a term “staple-free binding” will be used to refer to this method of binding a fibrous sheet bundle without the use of a staple.
However, according to these conventional sheet processing apparatuses, an increase in an applied force to fasten the sheets more securely results in the sheet bundle getting stuck to the teeth. The sheet bundle sticking to the teeth produces problems, such as, impeding conveyance of the sheet bundle to be presented to a user for collection or to a next step in the printing process.